"Black hole" is the name given to a body that has gravity excessive
enough to prevent light from escaping. It does this by pulling in
the nether at a velocity equal to or more than "c", the velocity of
light.

The Schwarzschild radius (some spell the name Schwartzschild)
shows the necessary specifications for a black hole to exist.
The Schwarzschild radius need not be the radius of the body that is
producing the gravity. It is the distance from the center of
the body at which the velocity of the incoming nether equals "c".
When the velocity of the incoming nether equals "c", light
cannot escape. It is like a river with a current moving at
a velocity equal to what a motorboat can go. The boat stays
in one place.

When the velocity of the current is greater than the boat's
velocity, the boat is pulled backward. When the incoming nether velocity
exceeds "c", light cannot even get started.

The Schwarzschild radius can be derived from the equation for
escape velocity which is the same as nether velocity downward
for any given location within a gravity funnel.

R = radius
G = the gravitational constant
M = mass of the body producing the gravity funnel
c = the velocity of light
m = mass of a smaller body in the gravity funnel
g = gravity at a point within the gravity funnel
F = force

F = GMm/R2 is the equation using the
gravitational constant to show the force between two masses.

F = mg is the equation for force downward for body within
a gravity funnel.

From the above, F = mg = GMm/R2. So:
gm = GMm/R2
g = GM/R2

v = (2Rg)1/2 is the equation for escape velocity or
nether velocity downward at any point in a gravity funnel.

It is interesting to go to a website that shows how Karl Schwarzschild
originally derived this radius. He used Einstein's theory of gravity
as a beginning. Without knowing the true nature of gravity, he was
able to use a lot of complicated theory and equations to arrive at the
correct solution which the physicists called his "metric" or the
"Schwarzschild solution" so that they could introduce another barrier
between themselves and the lay person.

Considering how little Schwarzschild knew of the true nature of gravity
when he derived his theory in 1916, he did a great job. But notice the
difference when one understands what gravity actually is. Once one knows
the true nature of gravity, the simple equation for escape velocity
along with the equation for the gravitational constant are enough to
complete the derivation in the form shown above.

The Schwarzschild radius was placed here to show how black hole radii
can be found within certain limits (these radii will always be equal to or
less than their corresponding Schwarzschild radii) - and for some other calculations to be
shown regarding the electron.
I did not bother with the above derivation before now because the
equation for escape velocity is so adequate, that a separate derivation
seemed too simple and too trivial to be shown.

Now I realize that part of nether theory's problem in being accepted
has to do with it (1) not being complicated enough to impress
and confuse the intelligent lay person, and (2) not showing
the precise derivations for things already accepted. Then there were
comments on why I did not choose to use calculus. Obviously, the
simple solution above is more elegant than the original two plus pages
of calculus that Schwarzschild used in his derivation.

There is a theory that after enough energy and matter (which is
energy also - but in a different form) enter a black hole,
they begin to "evaporate" (Stephen Hawking). This makes sense if
all the facts are not known. However, in my opinion, the observations
of Halton Arp (Seyfert galaxies) indicate that something else happens.
Of course, Halton Arp has not been allowed by the majority of
astronomers and astrophysicists to properly air his findings because
they upset the theories of some of the controlling prestigious members
of their club.

Further Explanation

Just in case you came upon this page without knowing what prompted it
to be here, you may click upon "Main Menu" to find the answers.

There are two major schools of thought in today's physics community.
The first consists of the old 20th century concepts which still attempt to justify
Einstein's relativity, spacetime with empty space, rubber-sheet gravity, and several
major paradoxes.
The second is the 21st century concepts of three-dimensional space filled with
dynamic ether (nether) moving along a fourth-dimensional time-line, gravity of accelerating
inward flow of dynamic ether, and relativity based upon dynamic ether.
This website
explains in detail the 21st century concepts. Another place to go for information on
these is the on-line The General Science Journal with essays written by
Lew Paxton Price.

According to the physics of dynamic ether, the following is true.

1. All celestial bodies have gravity of inflowing and accelerating dynamic ether.

2. These celestial bodies range from those with weak gravity to those with gravity
strong enough to prevent light (or anything else) from escaping.

2. The distance between the center of a body preventing the escape of light and a
point at which its inflow is equal to the speed of light is the Schwarzschild radius.
This phenomenon is similiar to the current in a river being strong enough to prevent a
motor boat from moving upstream. A black hole is called a "black hole" because it is black (not
allowing light to escape).

3. The volume inside the Schwarzschild radius is the volume of the black hole, and
should not be confused with the body creating the black hole, although that body is called a
black hole on occasion even though it is much smaller than the actual black hole that it
creates.

4. The body creating the strong gravity is a celestial body rather than a singularity or a
point without dimension. When such a body becomes strong enough, it shows itself as a
Seyfert galaxy, "burping" periodically when it becomes too full of matter. Go to the main menu
and click on "Glossary" to find more about Seyfert galaxies.